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Abstract

Broadly tunable multiple high-harmonic conical beams have been generated by means of a multistep χ(2) cascade processes in a two dimensional nonlinear photonic crystal. The nonlinear structure consists of a square lattice of inverted hexagonal domains with diameters and distances between domains as low as 1 μm. The large number of reciprocal lattice vectors provided by both the square nonlinear structure and the hexagonal shaped domains, along with imperfections on the size and shape of the individual domains make possible the simultaneous generation of second up to fifth harmonic conical beams in a single nonlinear structure by using different types of phase matching geometries. The frequency response can be tuned in an extremely large spectral range, and continuous generation of nonlinear conical beams covering the whole visible spectral region can be achieved. Further, the same photon energy can be generated at different orders, so that concentrically emitted conical beams with angular dispersion as large as Δθ = 50° can be observed. The results highlight the significance of highly controlled engineered 2D nonlinear structures to generate advanced multi-photon devices with large spatial and spectral tunable response.

Figures (5)

Fig. 1 (a). Optical picture of a square lattice of inverted ferroelectric domains in LiNbO3. The period of the structure is Λ~2 μm. (b) Schematic of the experimental set-up for conical high harmonic generation process for an incident fundamental beam along the z axis of the crystal. (c-f) Experimental images of multiple high-harmonic conical beams recorded in the far field when the fundamental wavelength was fixed at 1450, 1900, 2100 and 2300nm, respectively.

Fig. 2 (a). Schematic diagram showing the longitudinal phase matching condition of the multistep cascaded χ(2) –Cerenkov harmonic generation via Type I and Type II processes for the different harmonics observed in this work. (b) Multicolor ring-shaped Cerenkov multiple high harmonic generation recorded in the far field when the fundamental beam was tuned at 2100 nm. Red, green and blue conical emissions are simultaneously obtained via multistep frequency tripling, quadrupling and quintupling of the fundamental wave. (c) Angular dependence of the generated harmonic beams as a function of the fundamental wavelength. Theoretical calculations and experimental points are represented by solid lines and dots respectively.

Fig. 5 Effect of the polarization of the fundamental beam on the generated nonlinear patterns.(a-d) Third and fourth Cerenkov harmonic waves for different linear polarization angles. From left to right: γ = 0°, 45°, 90° and 135°, respectively. (e-f) Far field nonlinear patterns obtained for linearly polarized fundamental beams with polarization states parallel to x (γ = 0°) and y axis (γ = 90°) of the crystal. Fourth and fifth Cerenkov rings along with a set of inner rings arising from NLRN diffraction processes can be distinguished.